Electric vehicles are receiving considerable attention as a substitute for present gasoline-fueled vehicles. This interest is based primarily on zero atmospheric emissions obtainable from an all-electric vehicle. Several states are considering stricter emissions regulations for vehicles, and California has adopted regulations that will require zero emissions for a percentage of vehicles in certain urban areas. Electric vehicles also offer other advantages including reducing dependency on imported oil, since utilities in the United States generate a large portion of their energy demands using coal, gas, nuclear, and hydroelectric sources.
Even hybrid electric vehicles, such as those incorporating a small gasoline engine running at a constant speed to recharge an electric traction battery, offer anticipated lower emissions. See, for example, U.S. Pat. No. 4,351,405 to Fields et al. which discloses a hybrid vehicle including a gasoline engine for driving the front wheels during high speed and long distance driving, while the rear wheels are connected to electric motors for low speed and stop and go driving.
To obtain widespread acceptance as a suitable substitute for conventional gasoline-fueled vehicles, an electric vehicle should desirably mimic the operation of such a conventional gasoline vehicle, especially the drive train including a conventional automatic transmission. Functions such as braking and acceleration are readily controlled in an electric vehicle through a conventional brake pedal and accelerator pedal. The selection of drive, reverse, park, and neutral positions are also imitated on an electric vehicle. However, in a conventional gasoline vehicle, the engine always rotates in a same direction and the vehicle includes a clutch or fluid coupling to transmit torque to the vehicle wheels. In other words, a conventional automatic transmission will tend to start the vehicle moving forward, or creep, on a level surface once the driver releases the brake with the engine idling.
Moreover, starts upward on an incline are readily accommodated in a gasoline vehicle because the creep of the automatic transmission compensates for vehicle rollback during the time from when the driver releases the brake until the driver can depress the accelerator. In addition, even a conventional vehicle with a standard transmission includes numerous contributors of rolling friction which have a tendency to reduce the speed of rollback when starting upward on an incline.
An electric vehicle may have one or more electric motors directly driving the wheels as disclosed in U.S. Pat. No. 4,913,258 to Sakuri et al. Alternately, an electric vehicle may have an electric motor driving a set of wheels through a gearbox and differential. Since the electric motor does not typically "idle" as a conventional gasoline engine, a typical electric vehicle has a tendency to first roll backwards when starting upward on an incline. This rollback is particularly troublesome in traffic where the vehicle may roll backwards into the vehicle behind. In addition, when an electric vehicle rolls backward on a grade, the motor is rotating in a direction opposite to the desired direction of travel, and gear lash and drive shaft spring must first be taken up from the drive train before the vehicle may begin to move forward.
Current battery technology limits the driving range of an electric vehicle to be considerably less than a gasoline vehicle. Accordingly, much development has gone into reducing rolling friction of the tires and various mechanical train drive components of an electric vehicle to thereby improve efficiency and increase the driving range. Unfortunately, the reduced rolling friction exacerbates the problem of starting upward on an incline, since the electric vehicle has a tendency to roll back even faster.
Battery powered vehicles, such as automobiles, forklifts, and other utility vehicles, typically include some form of motor control logic that may assist in starting the vehicle when the vehicle is positioned on a grade. Creep, as in a gasoline vehicle, has been simulated in an electric forklift for operation on a level surface as shown in the graph of vehicle speed versus motor torque of FIG. 1, wherein a forward direction of intended movement is selected. The forklift control computer applies power to the motor to generate a predetermined constant output torque indicated by Point A on the graph, irrespective of the vehicle speed.
It has also been common in electric vehicles to limit the rate at which torque output can be increased from the electric motor to provide for a smooth and jerk-free start. In other words, to compensate for the lack of a clutch or fluid coupling, such as a torque converter in a conventional gasoline-fueled vehicle, a timed ramp on either the motor terminal voltage or armature current has been used to regulate the rate of torque increase from the motor of an electric vehicle.
Unfortunately, the timed ramp function is undesirable for starting on a grade. For example, in order to limit the jerkiness when starting on a level surface, the timed ramp is set to a relatively slow initial setting so that torque does not build up too quickly; however, with this same setting, substantial rollback may still occur on a grade because of the slowness of increasing torque.